The invention relates to a device for selectively cooling motor vehicle engine exhaust gases, fitted into the path of the exhaust gases in an exhaust line of the vehicle.
The exhaust lines of motor vehicles may comprise gas treatment devices, such as cleaning systems in which there are removed certain pollutants, such as nitrogen oxides (NOx), contained in the exhaust gases coming from the vehicle engine.
Treatment of the exhaust gases in order to remove pollutants consists in bringing the exhaust gases into contact, in a treatment device, with an active element composed of a mixture of different metals chosen for their capacity to catalyze conversion reactions of the pollutants or to adsorb the pollutants. The active element is deposited in the form of an impregnation layer on a porous ceramics support fixed inside a sheet metal casing of the cleaning device. The porous ceramics support is itself covered with a porous aluminum-based layer called the xe2x80x9cwash-coatxe2x80x9d, which is intended to increase considerably the contact surface between the gases and the active elements deposited on the wash-coat. The active element of the cleaning system and the wash-coat are not resistant above a certain temperature of the exhaust gases and, when in contact with exhaust gases at an excessive temperature, undergo irreversible damage, so that they are no longer capable of performing their cleaning function.
For example, in the case of cleaning systems for removing nitrogen oxides from the exhaust gases of direct injection type petrol engines, such cleaning systems generally being called xe2x80x9cNOx trapsxe2x80x9d, the limit temperature that can be withstood by the elements of the NOx trap in contact with the exhaust gases is currently of the order of 850xc2x0 C.
Those two conditions, which define the position of the cleaning elements in the exhaust line, are often incompatible.
It is then necessary, in order to fulfil the two conditions relating to the temperature of the exhaust gases in the cleaning system, which conditions may be incompatible, to provide a device for cooling the exhaust gases that is able to operate selectively under first and second cooling conditions.
In order to solve that problem, there has been proposed (e.g., in EP-03848.142) a cooling device that comprises first and second tubes for passage of the exhaust gases, the outer wall of which tubes is in contact with cooling air, for example atmospheric air, during operation of the vehicle engine. One of the tubes, which is of great length, permits the removal of a maximum amount of energy, while the second tube, which is of very reduced length, permits the removal of only a very small amount of thermal energy. A control valve allows the exhaust gases to be passed either into one tube or into the other, as a function of operating conditions of the engine or of the exhaust line. By adapting the lengths of the two tubes according to the desired operating conditions, it is possible to adjust the temperature of the exhaust gases before they enter the cleaning system, under all operating conditions of the engine.
However, that solution may require the use of very long tubes which cannot easily be accommodated in the vehicle, either in the engine block or in the sub-floor area.
The object of the invention is, therefore, to propose a device for cooling motor vehicle engine exhaust gases, fitted into the path of the exhaust gases, in an exhaust line of the vehicle, which device comprises a first gas flow path in at least one first pipe that is in contact by way of its outer wall with cooling air, and a second gas flow path that has a length greater than the length of the first flow path and is constituted by at least one second pipe having an outer wall in contact with cooling air, and means for directing the exhaust gases towards the first or second flow path, which means are controlled as a function of an operating parameter of the exhaust line or of the engine, which cooling device can easily be installed in the exhaust line of the engine, for example in a sub-floor area of the motor vehicle.
Those two conditions, which define the position of the cleaning elements in the exhaust line, are often incompatible.
It is then necessary, in order to fulfil the two conditions relating to the temperature of the exhaust gases in the cleaning system, which conditions may be incompatible, to provide a device for cooling the exhaust gases that is able to operate selectively under first and second cooling conditions.
In order to solve that problem, there has been proposed, for example in EP-0.848.142, a cooling device that comprises first and second tubes for passage of the exhaust gases, the outer wall of which tubes is in contact with cooling air, for example atmospheric air, during operation of the vehicle engine. One of the tubes, which is of great length, permits the removal of a maximum amount of energy, while the second tube, which is of very reduced length, permits the removal of only a very small amount of thermal energy. A control valve allows the exhaust gases to be passed either into one tube or into the other, as a function of operating conditions of the engine or of the exhaust line. By adapting the lengths of the two tubes according to the desired operating conditions, it is possible to adjust the temperature of the exhaust gases before they enter the cleaning system, under all operating conditions of the engine.
However, that solution may require the use of very long tubes which cannot easily be accommodated in the vehicle, either in the engine block or in the sub-floor area.
The object of the invention is, therefore, to propose a device for cooling motor vehicle engine exhaust gases, fitted into the path of the exhaust gases, in an exhaust line of the vehicle, which device comprises a first gas flow path in at least one first pipe that is in contact by way of its outer wall with cooling air, and a second gas flow path that has a length greater than the length of the first flow path and is constituted by at least one second pipe having an outer wall in contact with cooling air, and means for directing the exhaust gases totally towards the first or at least principally towards the second flow path, which means are controlled as a function of a thermal operating parameter of the exhaust line or of the engine, which cooling device can easily be installed in the exhaust line of the engine, for example in a sub-floor area of the motor vehicle.
To that end, the cooling device according to the invention comprises, in an outer casing that is substantially cylindrical in shape and through the lateral wall of which there pass openings, in which casing there are arranged the first path constituted by a tube extending in an axial direction of the cylindrical casing, and the second flow path constituted by a plurality of tubes fixed by holding means in substantially parallel arrangements, in an axial direction of the cylindrical casing, a pipe for the admission of exhaust gases into the cylindrical casing, which pipe is connected to an upstream part of the exhaust line, an exhaust gas evacuation pipe connected to a downstream part of the exhaust line, and exhaust gas distribution means arranged inside the cylindrical casing and connecting the admission pipe with end intake parts of the first and second paths, connecting end outlet parts of the first and second paths with the evacuation pipe, and connecting together intermediate parts of the second path constituted by tubes of the plurality of tubes, the directing means being constituted by at least one control valve for opening and closing an end intake part of at least one of the flow paths.
For the purposes of better understanding of the invention, an embodiment of a cooling device according to the invention will be described by way of example, with reference to the attached FIGS.